1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Spring with a mass at an angle, need original length of spring

  1. Mar 16, 2012 #1
    1. The problem statement, all variables and given/known data

    The Setup is depicted in the Attachment. Essentially there is a 5kg Mass on an incline (45). The incline measures 0.4m from the mass to the corner were it turns 90 degrees and measures 0.3m to were there is a spring (k = 20N/m. This spring attaches to the Mass. This system is in equilibrium with the spring stretched out as it is.

    The question is what is the length of the Spring not stretched. As in if the mass was un attached and the spring was at rest?

    2. Relevant equations
    Only relevant equation i can think of is F = k(λx) Basic Hookes law

    3. The attempt at a solution

    So first thing I did
    5* 9.8 = 49N

    Then 49cos(45) to get Fx of mass = 34.65N this should equal the X component of the force of the spring.

    Second the spring should measure 0.5 since it makes a triangle right triangle with 0.4 and 0.3 this also means the angle its at in relation to the x component of force is 36.87

    so F of the spring should be Fx = F cos(36.87)

    34.65N / cos( 36.87) = 43.31N

    then F = k λx and F/k = λx

    43.31/20 = 2.16m

    Now if the spring stretch measures 0.5m I dont see how λx can equal 2.16m

    That would mean that the spring has a negative measurement when unstretched and that doesn't make any sense.

    So help me out, were did I go wrong.

    Attached Files:

  2. jcsd
  3. Mar 16, 2012 #2


    User Avatar
    Homework Helper

    I wonder if k was supposed to equal 20 kN/m
  4. Mar 16, 2012 #3
    correction !!! sorry
  5. Mar 16, 2012 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Draw a Free Body Diagram for the 5 kg block.

  6. Mar 20, 2012 #5
    Ive checked with everybody. And so far what people have been able to tell me is that it just looks like the question has to be wrong

    I did draw the diagram, its how i saw were all the forces were going. Unless I messed it up somehow.
    Last edited: Mar 20, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook